I think like with most tag terms, we can use the first sentence or two of the corresponding Wikipedia article as a summary:
A workflow consists of a sequence of connected steps. Emphasis is on the flow paradigm, where each step follows the precedent without delay or gap and ends just before the subsequent step may begin.
If we don't want to lift from Wikipedia or ESRI, how about this definition:
In the context of GIS, a workflow describes a series of steps involved in collecting, deriving, refining, or disseminating information using GIS-related software and/or hardware, with an emphasis on the arrangement, selection, or order of such steps.
Some example questions (just making these up, they may or may not have existing equivalents):
- What is the process for collecting data in the field and storing it in a centralized database?
- What are the advantages and disadvantages of multi-user editing workflows in SDE?
- What is the workflow for publishing data to ArcGIS Online?
I think the workflow tag is applicable when the question is explicitly about a series of steps, and particularly the order or arrangement of such steps, and not necessarily just because the question is in the context of a workflow-like tool like wmx (Workflow Manager Extension for ArcGIS) or fme (Feature Manipulation Engine).
A related concept is a toolchain, often used in software development to define the series of tools used to build a software product.